GDB Remote Serial Protocol
Overview
There may be occasions when you need to know something about the protocol--for example, if there is only one serial port to your target machine, you might want your program to do something special if it recognizes a packet meant for GDB.
In the examples below, `->' and `<-' are used to indicate transmitted and received data respectfully.
All GDB commands and responses (other than acknowledgments) are sent as a packet. A packet is introduced with the character `$', the actual packet-data, and the terminating character `#' followed by a two-digit checksum:
$
packet-data#
checksum
The two-digit checksum is computed as the modulo 256 sum of all characters between the leading `$' and the trailing `#' (an eight bit unsigned checksum).
Implementors should note that prior to GDB 5.0 the protocol specification also included an optional two-digit sequence-id:
$
sequence-id:
packet-data#
checksum
That sequence-id was appended to the acknowledgment. GDB has never output sequence-ids. Stubs that handle packets added since GDB 5.0 must not accept sequence-id.
When either the host or the target machine receives a packet, the first response expected is an acknowledgment: either `+' (to indicate the package was received correctly) or `-' (to request retransmission):
->$
packet-data#
checksum <-+
The host (GDB) sends commands, and the target (the debugging stub incorporated in your program) sends a response. In the case of step and continue commands, the response is only sent when the operation has completed (the target has again stopped).
packet-data consists of a sequence of characters with the exception of `#' and `$' (see `X' packet for additional exceptions).
Fields within the packet should be separated using `,' `;' or `:'. Except where otherwise noted all numbers are represented in HEX with leading zeros suppressed.
Implementors should note that prior to GDB 5.0, the character `:' could not appear as the third character in a packet (as it would potentially conflict with the sequence-id).
Response data can be run-length encoded to save space. A `*'
means that the next character is an ASCII encoding giving a repeat count
which stands for that many repetitions of the character preceding the
`*'. The encoding is n+29
, yielding a printable character
where n >=3
(which is where rle starts to win). The printable
characters `$', `#', `+' and `-' or with a numeric
value greater than 126 should not be used.
So:
"0*
"
means the same as "0000".
The error response returned for some packets includes a two character error number. That number is not well defined.
For any command not supported by the stub, an empty response (`$#00') should be returned. That way it is possible to extend the protocol. A newer GDB can tell if a packet is supported based on that response.
A stub is required to support the `g', `G', `m', `M', `c', and `s' commands. All other commands are optional.
Packets
The following table provides a complete list of all currently defined commands and their corresponding response data. See section File-I/O remote protocol extension, for details about the File I/O extension of the remote protocol.
!
-- extended mode-
Enable extended mode. In extended mode, the remote server is made
persistent. The `R' packet is used to restart the program being
debugged.
Reply:
- `OK'
- The remote target both supports and has enabled extended mode.
?
-- last signal- Indicate the reason the target halted. The reply is the same as for step and continue. Reply: See section Stop Reply Packets, for the reply specifications.
a
-- reserved- Reserved for future use.
A
arglen,
argnum,
arg,...
-- set program arguments (reserved)-
Initialized `argv[]' array passed into program. arglen
specifies the number of bytes in the hex encoded byte stream arg.
See
gdbserver
for more details. Reply:- `OK'
- `ENN'
b
baud -- set baud (deprecated)- Change the serial line speed to baud. JTC: When does the transport layer state change? When it's received, or after the ACK is transmitted. In either case, there are problems if the command or the acknowledgment packet is dropped. Stan: If people really wanted to add something like this, and get it working for the first time, they ought to modify ser-unix.c to send some kind of out-of-band message to a specially-setup stub and have the switch happen "in between" packets, so that from remote protocol's point of view, nothing actually happened.
B
addr,mode -- set breakpoint (deprecated)- Set (mode is `S') or clear (mode is `C') a breakpoint at addr. This packet has been replaced by the `Z' and `z' packets (@xref{insert breakpoint or watchpoint packet}).
c
addr -- continue- addr is address to resume. If addr is omitted, resume at current address. Reply: See section Stop Reply Packets, for the reply specifications.
C
sig;
addr -- continue with signal-
Continue with signal sig (hex signal number). If
;
addr is omitted, resume at same address. Reply: See section Stop Reply Packets, for the reply specifications. d
-- toggle debug (deprecated)- Toggle debug flag.
D
-- detach-
Detach GDB from the remote system. Sent to the remote target
before GDB disconnects via the
detach
command. Reply:- `OK'
- for success
- `ENN'
- for an error
e
-- reserved- Reserved for future use.
E
-- reserved- Reserved for future use.
f
-- reserved- Reserved for future use.
F
RC,
EE,
CF;
XX -- Reply to target's F packet.-
This packet is send by GDB as reply to a
F
request packet sent by the target. This is part of the File-I/O protocol extension. See section File-I/O remote protocol extension, for the specification. g
-- read registers-
@anchor{read registers packet}
Read general registers.
Reply:
- `XX...'
-
Each byte of register data is described by two hex digits. The bytes
with the register are transmitted in target byte order. The size of
each register and their position within the `g' packet are
determined by the GDB internal macros
DEPRECATED_REGISTER_RAW_SIZE and REGISTER_NAME macros. The
specification of several standard
g
packets is specified below. - `ENN'
- for an error.
G
XX... -- write regs-
@xref{read registers packet}, for a description of the XX...
data.
Reply:
- `OK'
- for success
- `ENN'
- for an error
h
-- reserved- Reserved for future use.
H
ct... -- set thread-
Set thread for subsequent operations (`m', `M', `g',
`G', et.al.). c depends on the operation to be performed: it
should be `c' for step and continue operations, `g' for other
operations. The thread designator t... may be -1, meaning all
the threads, a thread number, or zero which means pick any thread.
Reply:
- `OK'
- for success
- `ENN'
- for an error
i
addr,
nnn -- cycle step (draft)-
@anchor{cycle step packet}
Step the remote target by a single clock cycle. If
,
nnn is present, cycle step nnn cycles. If addr is present, cycle step starting at that address. I
-- signal then cycle step (reserved)- @xref{step with signal packet}. @xref{cycle step packet}.
j
-- reserved- Reserved for future use.
J
-- reserved- Reserved for future use.
k
-- kill request- FIXME: There is no description of how to operate when a specific thread context has been selected (i.e. does 'k' kill only that thread?).
K
-- reserved- Reserved for future use.
l
-- reserved- Reserved for future use.
L
-- reserved- Reserved for future use.
m
addr,
length -- read memory-
Read length bytes of memory starting at address addr.
Neither GDB nor the stub assume that sized memory transfers are
assumed using word aligned accesses. FIXME: A word aligned memory
transfer mechanism is needed.
Reply:
- `XX...'
- XX... is mem contents. Can be fewer bytes than requested if able to read only part of the data. Neither GDB nor the stub assume that sized memory transfers are assumed using word aligned accesses. FIXME: A word aligned memory transfer mechanism is needed.
- `ENN'
- NN is errno
M
addr,length:
XX... -- write mem-
Write length bytes of memory starting at address addr.
XX... is the data.
Reply:
- `OK'
- for success
- `ENN'
- for an error (this includes the case where only part of the data was written).
n
-- reserved- Reserved for future use.
N
-- reserved- Reserved for future use.
o
-- reserved- Reserved for future use.
O
-- reservedp
hex number of register -- read register packet-
@xref{read registers packet}, for a description of how the returned
register value is encoded.
Reply:
- `XX...'
- the register's value
- `ENN'
- for an error
- `'
- Indicating an unrecognized query.
P
n...=
r... -- write register-
@anchor{write register packet}
Write register n... with value r..., which contains two hex
digits for each byte in the register (target byte order).
Reply:
- `OK'
- for success
- `ENN'
- for an error
q
query -- general query-
@anchor{general query packet}
Request info about query. In general GDB queries have a
leading upper case letter. Custom vendor queries should use a company
prefix (in lower case) ex: `qfsf.var'. query may optionally
be followed by a `,' or `;' separated list. Stubs must ensure
that they match the full query name.
Reply:
- `XX...'
- Hex encoded data from query. The reply can not be empty.
- `ENN'
- error reply
- `'
- Indicating an unrecognized query.
Q
var=
val -- general set- Set value of var to val. @xref{general query packet}, for a discussion of naming conventions.
r
-- reset (deprecated)- Reset the entire system.
R
XX -- remote restart-
Restart the program being debugged. XX, while needed, is ignored.
This packet is only available in extended mode.
Reply:
- `no reply'
- The `R' packet has no reply.
s
addr -- step- addr is address to resume. If addr is omitted, resume at same address. Reply: See section Stop Reply Packets, for the reply specifications.
S
sig;
addr -- step with signal- @anchor{step with signal packet} Like `C' but step not continue. Reply: See section Stop Reply Packets, for the reply specifications.
t
addr:
PP,
MM -- search- Search backwards starting at address addr for a match with pattern PP and mask MM. PP and MM are 4 bytes. addr must be at least 3 digits.
T
XX -- thread alive-
Find out if the thread XX is alive.
Reply:
- `OK'
- thread is still alive
- `ENN'
- thread is dead
u
-- reserved- Reserved for future use.
U
-- reserved- Reserved for future use.
v
-- verbose packet prefix-
Packets starting with
v
are identified by a multi-letter name, up to the first;
or?
(or the end of the packet). vCont
[;action[:
tid]]... -- extended resume-
Resume the inferior. Different actions may be specified for each thread.
If an action is specified with no tid, then it is applied to any
threads that don't have a specific action specified; if no default action is
specified then other threads should remain stopped. Specifying multiple
default actions is an error; specifying no actions is also an error.
Thread IDs are specified in hexadecimal. Currently supported actions are:
c
- Continue.
Csig
- Continue with signal sig. sig should be two hex digits.
s
- Step.
Ssig
- Step with signal sig. sig should be two hex digits.
vCont
. Reply: See section Stop Reply Packets, for the reply specifications. vCont?
-- extended resume query-
Query support for the
vCont
packet. Reply:- `
vCont
[;action]...' -
The
vCont
packet is supported. Each action is a supported command in thevCont
packet. - `'
-
The
vCont
packet is not supported.
- `
V
-- reserved- Reserved for future use.
w
-- reserved- Reserved for future use.
W
-- reserved- Reserved for future use.
x
-- reserved- Reserved for future use.
X
addr,
length:XX... -- write mem (binary)-
addr is address, length is number of bytes, XX...
is binary data. The characters
$
,#
, and0x7d
are escaped using0x7d
, and then XORed with0x20
. For example,0x7d
would be transmitted as0x7d 0x5d
. Reply:- `OK'
- for success
- `ENN'
- for an error
y
-- reserved- Reserved for future use.
Y
reserved- Reserved for future use.
z
type,
addr,
length -- remove breakpoint or watchpoint (draft)Z
type,
addr,
length -- insert breakpoint or watchpoint (draft)-
@anchor{insert breakpoint or watchpoint packet}
Insert (
Z
) or remove (z
) a type breakpoint or watchpoint starting at address address and covering the next length bytes. Each breakpoint and watchpoint packet type is documented separately. Implementation notes: A remote target shall return an empty string for an unrecognized breakpoint or watchpoint packet type. A remote target shall support either both or neither of a givenZ
type... andz
type... packet pair. To avoid potential problems with duplicate packets, the operations should be implemented in an idempotent way. z
0
,
addr,
length -- remove memory breakpoint (draft)Z
0
,
addr,
length -- insert memory breakpoint (draft)-
Insert (
Z0
) or remove (z0
) a memory breakpoint at addressaddr
of sizelength
. A memory breakpoint is implemented by replacing the instruction at addr with a software breakpoint or trap instruction. Thelength
is used by targets that indicates the size of the breakpoint (in bytes) that should be inserted (e.g., the ARM and MIPS can insert either a 2 or 4 byte breakpoint). Implementation note: It is possible for a target to copy or move code that contains memory breakpoints (e.g., when implementing overlays). The behavior of this packet, in the presence of such a target, is not defined. Reply:- `OK'
- success
- `'
- not supported
- `ENN'
- for an error
z
1
,
addr,
length -- remove hardware breakpoint (draft)Z
1
,
addr,
length -- insert hardware breakpoint (draft)-
Insert (
Z1
) or remove (z1
) a hardware breakpoint at addressaddr
of sizelength
. A hardware breakpoint is implemented using a mechanism that is not dependant on being able to modify the target's memory. Implementation note: A hardware breakpoint is not affected by code movement. Reply:- `OK'
- success
- `'
- not supported
- `ENN'
- for an error
z
2
,
addr,
length -- remove write watchpoint (draft)Z
2
,
addr,
length -- insert write watchpoint (draft)-
Insert (
Z2
) or remove (z2
) a write watchpoint. Reply:- `OK'
- success
- `'
- not supported
- `ENN'
- for an error
z
3
,
addr,
length -- remove read watchpoint (draft)Z
3
,
addr,
length -- insert read watchpoint (draft)-
Insert (
Z3
) or remove (z3
) a read watchpoint. Reply:- `OK'
- success
- `'
- not supported
- `ENN'
- for an error
z
4
,
addr,
length -- remove access watchpoint (draft)Z
4
,
addr,
length -- insert access watchpoint (draft)-
Insert (
Z4
) or remove (z4
) an access watchpoint. Reply:- `OK'
- success
- `'
- not supported
- `ENN'
- for an error
Stop Reply Packets
The `C', `c', `S', `s' and `?' packets can receive any of the below as a reply. In the case of the `C', `c', `S' and `s' packets, that reply is only returned when the target halts. In the below the exact meaning of `signal number' is poorly defined. In general one of the UNIX signal numbering conventions is used.
- `SAA'
- AA is the signal number
- `
T
AAn...:
r...;
n...:
r...;
n...:
r...;
' -
AA = two hex digit signal number; n... = register number
(hex), r... = target byte ordered register contents, size defined
by
DEPRECATED_REGISTER_RAW_SIZE
; n... = `thread', r... = thread process ID, this is a hex integer; n... = (`watch' | `rwatch' | `awatch', r... = data address, this is a hex integer; n... = other string not starting with valid hex digit. GDB should ignore this n..., r... pair and go on to the next. This way we can extend the protocol. - `WAA'
- The process exited, and AA is the exit status. This is only applicable to certain targets.
- `XAA'
- The process terminated with signal AA.
- `OXX...'
- XX... is hex encoding of ASCII data. This can happen at any time while the program is running and the debugger should continue to wait for `W', `T', etc.
- `Fcall-id
,
parameter...' -
call-id is the identifier which says which host system call should
be called. This is just the name of the function. Translation into the
correct system call is only applicable as it's defined in GDB.
See section File-I/O remote protocol extension, for a list of implemented
system calls.
parameter... is a list of parameters as defined for this very
system call.
The target replies with this packet when it expects GDB to call
a host system call on behalf of the target. GDB replies with
an appropriate
F
packet and keeps up waiting for the next reply packet from the target. The latest `C', `c', `S' or `s' action is expected to be continued. See section File-I/O remote protocol extension, for more details.
General Query Packets
The following set and query packets have already been defined.
q
C
-- current thread-
Return the current thread id.
Reply:
- `
QC
pid' - Where pid is an unsigned hexidecimal process id.
- `*'
- Any other reply implies the old pid.
- `
q
fThreadInfo
-- all thread ids-
q
sThreadInfo
Obtain a list of active thread ids from the target (OS). Since there may be too many active threads to fit into one reply packet, this query works iteratively: it may require more than one query/reply sequence to obtain the entire list of threads. The first query of the sequence will be theqf
ThreadInfo
query; subsequent queries in the sequence will be theqs
ThreadInfo
query. NOTE: replaces theqL
query (see below). Reply:- `
m
id' - A single thread id
- `
m
id,id...' - a comma-separated list of thread ids
- `
l
' - (lower case 'el') denotes end of list.
qs
form of the query), until the target responds withl
(lower-case el, for'last'
). - `
q
ThreadExtraInfo
,
id -- extra thread info-
Where id is a thread-id in big-endian hex. Obtain a printable
string description of a thread's attributes from the target OS. This
string may contain anything that the target OS thinks is interesting for
GDB to tell the user about the thread. The string is displayed
in GDB's `info threads' display. Some examples of
possible thread extra info strings are "Runnable", or "Blocked on
Mutex".
Reply:
- `XX...'
- Where XX... is a hex encoding of ASCII data, comprising the printable string containing the extra information about the thread's attributes.
q
L
startflagthreadcountnextthread -- query LIST or threadLIST (deprecated)-
Obtain thread information from RTOS. Where: startflag (one hex
digit) is one to indicate the first query and zero to indicate a
subsequent query; threadcount (two hex digits) is the maximum
number of threads the response packet can contain; and nextthread
(eight hex digits), for subsequent queries (startflag is zero), is
returned in the response as argthread.
NOTE: this query is replaced by the
q
fThreadInfo
query (see above). Reply:- `
q
M
countdoneargthreadthread...' -
Where: count (two hex digits) is the number of threads being
returned; done (one hex digit) is zero to indicate more threads
and one indicates no further threads; argthreadid (eight hex
digits) is nextthread from the request packet; thread...
is a sequence of thread IDs from the target. threadid (eight hex
digits). See
remote.c:parse_threadlist_response()
.
- `
q
CRC:
addr,
length -- compute CRC of memory block-
Reply:
- `
E
NN' - An error (such as memory fault)
- `
C
CRC32' - A 32 bit cyclic redundancy check of the specified memory region.
- `
q
Offsets
-- query sect offs-
Get section offsets that the target used when re-locating the downloaded
image. Note: while a
Bss
offset is included in the response, GDB ignores this and instead applies theData
offset to theBss
section. Reply:- `
Text=
xxx;Data=
yyy;Bss=
zzz'
- `
q
P
modethreadid -- thread info request-
Returns information on threadid. Where: mode is a hex
encoded 32 bit mode; threadid is a hex encoded 64 bit thread ID.
Reply:
- `*'
remote.c:remote_unpack_thread_info_response()
. q
Rcmd,
command -- remote command-
command (hex encoded) is passed to the local interpreter for
execution. Invalid commands should be reported using the output string.
Before the final result packet, the target may also respond with a
number of intermediate
O
output console output packets. Implementors should note that providing access to a stubs's interpreter may have security implications. Reply:- `OK'
- A command response with no output.
- `OUTPUT'
- A command response with the hex encoded output string OUTPUT.
- `
E
NN' - Indicate a badly formed request.
- ``''
- When `q'`Rcmd' is not recognized.
qSymbol::
-- symbol lookup-
Notify the target that GDB is prepared to serve symbol lookup
requests. Accept requests from the target for the values of symbols.
Reply:
- `
OK
' - The target does not need to look up any (more) symbols.
- `
qSymbol:
sym_name' -
The target requests the value of symbol sym_name (hex encoded).
GDB may provide the value by using the
qSymbol:
sym_value:sym_name message, described below.
- `
qSymbol:
sym_value:sym_name -- symbol value-
Set the value of sym_name to sym_value.
sym_name (hex encoded) is the name of a symbol whose value the
target has previously requested.
sym_value (hex) is the value for symbol sym_name. If
GDB cannot supply a value for sym_name, then this field
will be empty.
Reply:
- `
OK
' - The target does not need to look up any (more) symbols.
- `
qSymbol:
sym_name' - The target requests the value of a new symbol sym_name (hex encoded). GDB will continue to supply the values of symbols (if available), until the target ceases to request them.
- `
qPart
:object:read
:annex:offset,length -- read special data-
Read uninterpreted bytes from the target's special data area
identified by the keyword
object
. Request length bytes starting at offset bytes into the data. The content and encoding of annex is specific to the object; it can supply additional details about what data to access. Here are the specific requests of this form defined so far. All `qPart
:object:read
:...' requests use the same reply formats, listed below.qPart
:auxv
:read
::offset,length- Access the target's auxiliary vector. See section Operating system auxiliary information, and see section Remote configuration. Note annex must be empty.
OK
- The offset in the request is at the end of the data. There is no more data to be read.
- XX...
- Hex encoded data bytes read. This may be fewer bytes than the length in the request.
E00
- The request was malformed, or annex was invalid.
E
nn-
The offset was invalid, or there was an error encountered reading the data.
nn is a hex-encoded
errno
value. ""
(empty)- An empty reply indicates the object or annex string was not recognized by the stub.
qPart
:object:write
:annex:offset:data...-
Write uninterpreted bytes into the target's special data area
identified by the keyword
object
, starting at offset bytes into the data. data... is the hex-encoded data to be written. The content and encoding of annex is specific to the object; it can supply additional details about what data to access. No requests of this form are presently in use. This specification serves as a placeholder to document the common format that new specific request specifications ought to use. Reply:- nn
- nn (hex encoded) is the number of bytes written. This may be fewer bytes than supplied in the request.
E00
- The request was malformed, or annex was invalid.
E
nn-
The offset was invalid, or there was an error encountered writing the data.
nn is a hex-encoded
errno
value. ""
(empty)- An empty reply indicates the object or annex string was not recognized by the stub, or that the object does not support writing.
qPart
:object:operation:...- Requests of this form may be added in the future. When a stub does not recognize the object keyword, or its support for object does not recognize the operation keyword, the stub must respond with an empty packet.
qGetTLSAddr
:thread-id,offset,lm -- get thread local storage address-
Fetch the address associated with thread local storage specified
by thread-id, offset, and lm.
thread-id is the (big endian, hex encoded) thread id associated with the
thread for which to fetch the TLS address.
offset is the (big endian, hex encoded) offset associated with the
thread local variable. (This offset is obtained from the debug
information associated with the variable.)
lm is the (big endian, hex encoded) OS/ABI specific encoding of the
the load module associated with the thread local storage. For example,
a GNU/Linux system will pass the link map address of the shared
object associated with the thread local storage under consideration.
Other operating environments may choose to represent the load module
differently, so the precise meaning of this parameter will vary.
Reply:
- XX...
- Hex encoded (big endian) bytes representing the address of the thread local storage requested.
E
nn (where nn are hex digits)- An error occurred.
""
(empty)-
An empty reply indicates that
qGetTLSAddr
is not supported by the stub.
set remote get-thread-local-storage-address
command (see section Remote configuration).
Register Packet Format
The following `g'/`G' packets have previously been defined. In the below, some thirty-two bit registers are transferred as sixty-four bits. Those registers should be zero/sign extended (which?) to fill the space allocated. Register bytes are transfered in target byte order. The two nibbles within a register byte are transfered most-significant - least-significant.
- MIPS32
- All registers are transfered as thirty-two bit quantities in the order: 32 general-purpose; sr; lo; hi; bad; cause; pc; 32 floating-point registers; fsr; fir; fp.
- MIPS64
-
All registers are transfered as sixty-four bit quantities (including
thirty-two bit registers such as
sr
). The ordering is the same asMIPS32
.
Examples
Example sequence of a target being re-started. Notice how the restart does not get any direct output:
->R00
<-+
target restarts ->?
<-+
<-T001:1234123412341234
->+
Example sequence of a target being stepped by a single instruction:
->G1445...
<-+
->s
<-+
time passes <-T001:1234123412341234
->+
->g
<-+
<-1455...
->+
File-I/O remote protocol extension
File-I/O Overview
The File I/O remote protocol extension (short: File-I/O) allows the target to use the host's file system and console I/O when calling various system calls. System calls on the target system are translated into a remote protocol packet to the host system which then performs the needed actions and returns with an adequate response packet to the target system. This simulates file system operations even on targets that lack file systems.
The protocol is defined host- and target-system independent. It uses its own independent representation of datatypes and values. Both, GDB and the target's GDB stub are responsible for translating the system dependent values into the unified protocol values when data is transmitted.
The communication is synchronous. A system call is possible only when GDB is waiting for the `C', `c', `S' or `s' packets. While GDB handles the request for a system call, the target is stopped to allow deterministic access to the target's memory. Therefore File-I/O is not interuptible by target signals. It is possible to interrupt File-I/O by a user interrupt (Ctrl-C), though.
The target's request to perform a host system call does not finish the latest `C', `c', `S' or `s' action. That means, after finishing the system call, the target returns to continuing the previous activity (continue, step). No additional continue or step request from GDB is required.
(gdb) continue <- target requests 'system call X' target is stopped, GDB executes system call -> GDB returns result ... target continues, GDB returns to wait for the target <- target hits breakpoint and sends a Txx packet
The protocol is only used for files on the host file system and for I/O on the console. Character or block special devices, pipes, named pipes or sockets or any other communication method on the host system are not supported by this protocol.
Protocol basics
The File-I/O protocol uses the F
packet, as request as well
as as reply packet. Since a File-I/O system call can only occur when
GDB is waiting for the continuing or stepping target, the
File-I/O request is a reply that GDB has to expect as a result
of a former `C', `c', `S' or `s' packet.
This F
packet contains all information needed to allow GDB
to call the appropriate host system call:
- A unique identifier for the requested system call.
- All parameters to the system call. Pointers are given as addresses in the target memory address space. Pointers to strings are given as pointer/length pair. Numerical values are given as they are. Numerical control values are given in a protocol specific representation.
At that point GDB has to perform the following actions.
-
If parameter pointer values are given, which point to data needed as input
to a system call, GDB requests this data from the target with a
standard
m
packet request. This additional communication has to be expected by the target implementation and is handled as any otherm
packet. - GDB translates all value from protocol representation to host representation as needed. Datatypes are coerced into the host types.
- GDB calls the system call
- It then coerces datatypes back to protocol representation.
-
If pointer parameters in the request packet point to buffer space in which
a system call is expected to copy data to, the data is transmitted to the
target using a
M
orX
packet. This packet has to be expected by the target implementation and is handled as any otherM
orX
packet.
Eventually GDB replies with another F
packet which contains all
necessary information for the target to continue. This at least contains
- Return value.
-
errno
, if has been changed by the system call. - "Ctrl-C" flag.
After having done the needed type and value coercion, the target continues the latest continue or step action.
The F
request packet
The F
request packet has the following format:
call-id is the identifier to indicate the host system call to be called. This is just the name of the function. parameter... are the parameters to the system call.F
call-id,
parameter...
Parameters are hexadecimal integer values, either the real values in case of scalar datatypes, as pointers to target buffer space in case of compound datatypes and unspecified memory areas or as pointer/length pairs in case of string parameters. These are appended to the call-id, each separated from its predecessor by a comma. All values are transmitted in ASCII string representation, pointer/length pairs separated by a slash.
The F
reply packet
The F
reply packet has the following format:
retcode is the return code of the system call as hexadecimal value. errno is the errno set by the call, in protocol specific representation. This parameter can be omitted if the call was successful. Ctrl-C flag is only send if the user requested a break. In this case, errno must be send as well, even if the call was successful. The Ctrl-C flag itself consists of the character 'C':F
retcode,
errno,
Ctrl-C flag;
call specific attachment
F0,0,Cor, if the call was interupted before the host call has been performed:
F-1,4,Cassuming 4 is the protocol specific representation of
EINTR
.
Memory transfer
Structured data which is transferred using a memory read or write as e.g.
a struct stat
is expected to be in a protocol specific format with
all scalar multibyte datatypes being big endian. This should be done by
the target before the F
packet is sent resp. by GDB before
it transfers memory to the target. Transferred pointers to structured
data should point to the already coerced data at any time.
The Ctrl-C message
A special case is, if the Ctrl-C flag is set in the GDB
reply packet. In this case the target should behave, as if it had
gotten a break message. The meaning for the target is "system call
interupted by SIGINT
". Consequentially, the target should actually stop
(as with a break message) and return to GDB with a T02
packet. In this case, it's important for the target to know, in which
state the system call was interrupted. Since this action is by design
not an atomic operation, we have to differ between two cases:
- The system call hasn't been performed on the host yet.
- The system call on the host has been finished.
These two states can be distinguished by the target by the value of the
returned errno
. If it's the protocol representation of EINTR
, the system
call hasn't been performed. This is equivalent to the EINTR
handling
on POSIX systems. In any other case, the target may presume that the
system call has been finished -- successful or not -- and should behave
as if the break message arrived right after the system call.
GDB must behave reliable. If the system call has not been called
yet, GDB may send the F
reply immediately, setting EINTR
as
errno
in the packet. If the system call on the host has been finished
before the user requests a break, the full action must be finshed by
GDB. This requires sending M
or X
packets as they fit.
The F
packet may only be send when either nothing has happened
or the full action has been completed.
Console I/O
By default and if not explicitely closed by the target system, the file
descriptors 0, 1 and 2 are connected to the GDB console. Output
on the GDB console is handled as any other file output operation
(write(1, ...)
or write(2, ...)
). Console input is handled
by GDB so that after the target read request from file descriptor
0 all following typing is buffered until either one of the following
conditions is met:
-
The user presses Ctrl-C. The behaviour is as explained above, the
read
system call is treated as finished. - The user presses Enter. This is treated as end of input with a trailing line feed.
- The user presses Ctrl-D. This is treated as end of input. No trailing character, especially no Ctrl-D is appended to the input.
If the user has typed more characters as fit in the buffer given to
the read call, the trailing characters are buffered in GDB until
either another read(0, ...)
is requested by the target or debugging
is stopped on users request.
The `isatty' function call
A special case in this protocol is the library call isatty
which
is implemented as its own call inside of this protocol. It returns
1 to the target if the file descriptor given as parameter is attached
to the GDB console, 0 otherwise. Implementing through system calls
would require implementing ioctl
and would be more complex than
needed.
The `system' function call
The other special case in this protocol is the system
call which
is implemented as its own call, too. GDB is taking over the full
task of calling the necessary host calls to perform the system
call. The return value of system
is simplified before it's returned
to the target. Basically, the only signal transmitted back is EINTR
in case the user pressed Ctrl-C. Otherwise the return value consists
entirely of the exit status of the called command.
Due to security concerns, the system
call is by default refused
by GDB. The user has to allow this call explicitly with the
set remote system-call-allowed 1 command.
set remote system-call-allowed
-
Control whether to allow the
system
calls in the File I/O protocol for the remote target. The default is zero (disabled). show remote system-call-allowed
- Show the current setting of system calls for the remote File I/O protocol.
List of supported calls
open
Synopsis: int open(const char *pathname, int flags); int open(const char *pathname, int flags, mode_t mode); Request: Fopen,pathptr/len,flags,mode
flags
is the bitwise or of the following values:
O_CREAT
- If the file does not exist it will be created. The host rules apply as far as file ownership and time stamps are concerned.
O_EXCL
- When used with O_CREAT, if the file already exists it is an error and open() fails.
O_TRUNC
- If the file already exists and the open mode allows writing (O_RDWR or O_WRONLY is given) it will be truncated to length 0.
O_APPEND
- The file is opened in append mode.
O_RDONLY
- The file is opened for reading only.
O_WRONLY
- The file is opened for writing only.
O_RDWR
- The file is opened for reading and writing. Each other bit is silently ignored.
mode
is the bitwise or of the following values:
S_IRUSR
- User has read permission.
S_IWUSR
- User has write permission.
S_IRGRP
- Group has read permission.
S_IWGRP
- Group has write permission.
S_IROTH
- Others have read permission.
S_IWOTH
- Others have write permission. Each other bit is silently ignored.
Return value: open returns the new file descriptor or -1 if an error occured. Errors:
EEXIST
- pathname already exists and O_CREAT and O_EXCL were used.
EISDIR
- pathname refers to a directory.
EACCES
- The requested access is not allowed.
ENAMETOOLONG
- pathname was too long.
ENOENT
- A directory component in pathname does not exist.
ENODEV
- pathname refers to a device, pipe, named pipe or socket.
EROFS
- pathname refers to a file on a read-only filesystem and write access was requested.
EFAULT
- pathname is an invalid pointer value.
ENOSPC
- No space on device to create the file.
EMFILE
- The process already has the maximum number of files open.
ENFILE
- The limit on the total number of files open on the system has been reached.
EINTR
- The call was interrupted by the user.
close
Synopsis: int close(int fd); Request: Fclose,fd Return value: close returns zero on success, or -1 if an error occurred. Errors:
EBADF
- fd isn't a valid open file descriptor.
EINTR
- The call was interrupted by the user.
read
Synopsis: int read(int fd, void *buf, unsigned int count); Request: Fread,fd,bufptr,count Return value: On success, the number of bytes read is returned. Zero indicates end of file. If count is zero, read returns zero as well. On error, -1 is returned. Errors:
EBADF
- fd is not a valid file descriptor or is not open for reading.
EFAULT
- buf is an invalid pointer value.
EINTR
- The call was interrupted by the user.
write
Synopsis: int write(int fd, const void *buf, unsigned int count); Request: Fwrite,fd,bufptr,count Return value: On success, the number of bytes written are returned. Zero indicates nothing was written. On error, -1 is returned. Errors:
EBADF
- fd is not a valid file descriptor or is not open for writing.
EFAULT
- buf is an invalid pointer value.
EFBIG
- An attempt was made to write a file that exceeds the host specific maximum file size allowed.
ENOSPC
- No space on device to write the data.
EINTR
- The call was interrupted by the user.
lseek
Synopsis: long lseek (int fd, long offset, int flag); Request: Flseek,fd,offset,flag
flag
is one of:
SEEK_SET
- The offset is set to offset bytes.
SEEK_CUR
- The offset is set to its current location plus offset bytes.
SEEK_END
- The offset is set to the size of the file plus offset bytes.
Return value: On success, the resulting unsigned offset in bytes from the beginning of the file is returned. Otherwise, a value of -1 is returned. Errors:
EBADF
- fd is not a valid open file descriptor.
ESPIPE
- fd is associated with the GDB console.
EINVAL
- flag is not a proper value.
EINTR
- The call was interrupted by the user.
rename
Synopsis: int rename(const char *oldpath, const char *newpath); Request: Frename,oldpathptr/len,newpathptr/len Return value: On success, zero is returned. On error, -1 is returned. Errors:
EISDIR
- newpath is an existing directory, but oldpath is not a directory.
EEXIST
- newpath is a non-empty directory.
EBUSY
- oldpath or newpath is a directory that is in use by some process.
EINVAL
- An attempt was made to make a directory a subdirectory of itself.
ENOTDIR
- A component used as a directory in oldpath or new path is not a directory. Or oldpath is a directory and newpath exists but is not a directory.
EFAULT
- oldpathptr or newpathptr are invalid pointer values.
EACCES
- No access to the file or the path of the file.
ENAMETOOLONG
- oldpath or newpath was too long.
ENOENT
- A directory component in oldpath or newpath does not exist.
EROFS
- The file is on a read-only filesystem.
ENOSPC
- The device containing the file has no room for the new directory entry.
EINTR
- The call was interrupted by the user.
unlink
Synopsis: int unlink(const char *pathname); Request: Funlink,pathnameptr/len Return value: On success, zero is returned. On error, -1 is returned. Errors:
EACCES
- No access to the file or the path of the file.
EPERM
- The system does not allow unlinking of directories.
EBUSY
- The file pathname cannot be unlinked because it's being used by another process.
EFAULT
- pathnameptr is an invalid pointer value.
ENAMETOOLONG
- pathname was too long.
ENOENT
- A directory component in pathname does not exist.
ENOTDIR
- A component of the path is not a directory.
EROFS
- The file is on a read-only filesystem.
EINTR
- The call was interrupted by the user.
stat/fstat
Synopsis: int stat(const char *pathname, struct stat *buf); int fstat(int fd, struct stat *buf); Request: Fstat,pathnameptr/len,bufptr Ffstat,fd,bufptr Return value: On success, zero is returned. On error, -1 is returned. Errors:
EBADF
- fd is not a valid open file.
ENOENT
- A directory component in pathname does not exist or the path is an empty string.
ENOTDIR
- A component of the path is not a directory.
EFAULT
- pathnameptr is an invalid pointer value.
EACCES
- No access to the file or the path of the file.
ENAMETOOLONG
- pathname was too long.
EINTR
- The call was interrupted by the user.
gettimeofday
Synopsis: int gettimeofday(struct timeval *tv, void *tz); Request: Fgettimeofday,tvptr,tzptr Return value: On success, 0 is returned, -1 otherwise. Errors:
EINVAL
- tz is a non-NULL pointer.
EFAULT
- tvptr and/or tzptr is an invalid pointer value.
isatty
Synopsis: int isatty(int fd); Request: Fisatty,fd Return value: Returns 1 if fd refers to the GDB console, 0 otherwise. Errors:
EINTR
- The call was interrupted by the user.
system
Synopsis: int system(const char *command); Request: Fsystem,commandptr/len Return value: The value returned is -1 on error and the return status of the command otherwise. Only the exit status of the command is returned, which is extracted from the hosts system return value by calling WEXITSTATUS(retval). In case /bin/sh could not be executed, 127 is returned. Errors:
EINTR
- The call was interrupted by the user.
Protocol specific representation of datatypes
Integral datatypes
The integral datatypes used in the system calls are
int, unsigned int, long, unsigned long, mode_t and time_t
Int
, unsigned int
, mode_t
and time_t
are
implemented as 32 bit values in this protocol.
Long
and unsigned long
are implemented as 64 bit types.
See section Limits, for corresponding MIN and MAX values (similar to those in `limits.h') to allow range checking on host and target.
time_t
datatypes are defined as seconds since the Epoch.
All integral datatypes transferred as part of a memory read or write of a
structured datatype e.g. a struct stat
have to be given in big endian
byte order.
Pointer values
Pointers to target data are transmitted as they are. An exception is made for pointers to buffers for which the length isn't transmitted as part of the function call, namely strings. Strings are transmitted as a pointer/length pair, both as hex values, e.g.
1aaf/12
which is a pointer to data of length 18 bytes at position 0x1aaf. The length is defined as the full string length in bytes, including the trailing null byte. Example:
"hello, world" at address 0x123456
is transmitted as
123456/d
struct stat
The buffer of type struct stat used by the target and GDB is defined as follows:
struct stat { unsigned int st_dev; /* device */ unsigned int st_ino; /* inode */ mode_t st_mode; /* protection */ unsigned int st_nlink; /* number of hard links */ unsigned int st_uid; /* user ID of owner */ unsigned int st_gid; /* group ID of owner */ unsigned int st_rdev; /* device type (if inode device) */ unsigned long st_size; /* total size, in bytes */ unsigned long st_blksize; /* blocksize for filesystem I/O */ unsigned long st_blocks; /* number of blocks allocated */ time_t st_atime; /* time of last access */ time_t st_mtime; /* time of last modification */ time_t st_ctime; /* time of last change */ };
The integral datatypes are conforming to the definitions given in the approriate section (see section Integral datatypes, for details) so this structure is of size 64 bytes.
The values of several fields have a restricted meaning and/or range of values.
st_dev: 0 file 1 console st_ino: No valid meaning for the target. Transmitted unchanged. st_mode: Valid mode bits are described in Appendix C. Any other bits have currently no meaning for the target. st_uid: No valid meaning for the target. Transmitted unchanged. st_gid: No valid meaning for the target. Transmitted unchanged. st_rdev: No valid meaning for the target. Transmitted unchanged. st_atime, st_mtime, st_ctime: These values have a host and file system dependent accuracy. Especially on Windows hosts the file systems don't support exact timing values.
The target gets a struct stat of the above representation and is responsible to coerce it to the target representation before continuing.
Note that due to size differences between the host and target representation of stat members, these members could eventually get truncated on the target.
struct timeval
The buffer of type struct timeval used by the target and GDB is defined as follows:
struct timeval { time_t tv_sec; /* second */ long tv_usec; /* microsecond */ };
The integral datatypes are conforming to the definitions given in the approriate section (see section Integral datatypes, for details) so this structure is of size 8 bytes.
Constants
The following values are used for the constants inside of the protocol. GDB and target are resposible to translate these values before and after the call as needed.
Open flags
All values are given in hexadecimal representation.
O_RDONLY 0x0 O_WRONLY 0x1 O_RDWR 0x2 O_APPEND 0x8 O_CREAT 0x200 O_TRUNC 0x400 O_EXCL 0x800
mode_t values
All values are given in octal representation.
S_IFREG 0100000 S_IFDIR 040000 S_IRUSR 0400 S_IWUSR 0200 S_IXUSR 0100 S_IRGRP 040 S_IWGRP 020 S_IXGRP 010 S_IROTH 04 S_IWOTH 02 S_IXOTH 01
Errno values
All values are given in decimal representation.
EPERM 1 ENOENT 2 EINTR 4 EBADF 9 EACCES 13 EFAULT 14 EBUSY 16 EEXIST 17 ENODEV 19 ENOTDIR 20 EISDIR 21 EINVAL 22 ENFILE 23 EMFILE 24 EFBIG 27 ENOSPC 28 ESPIPE 29 EROFS 30 ENAMETOOLONG 91 EUNKNOWN 9999
EUNKNOWN is used as a fallback error value if a host system returns any error value not in the list of supported error numbers.
Lseek flags
SEEK_SET 0 SEEK_CUR 1 SEEK_END 2
Limits
All values are given in decimal representation.
INT_MIN -2147483648 INT_MAX 2147483647 UINT_MAX 4294967295 LONG_MIN -9223372036854775808 LONG_MAX 9223372036854775807 ULONG_MAX 18446744073709551615
File-I/O Examples
Example sequence of a write call, file descriptor 3, buffer is at target address 0x1234, 6 bytes should be written:
<-Fwrite,3,1234,6
request memory read from target ->m1234,6
<- XXXXXX return "6 bytes written" ->F6
Example sequence of a read call, file descriptor 3, buffer is at target address 0x1234, 6 bytes should be read:
<-Fread,3,1234,6
request memory write to target ->X1234,6:XXXXXX
return "6 bytes read" ->F6
Example sequence of a read call, call fails on the host due to invalid file descriptor (EBADF):
<-Fread,3,1234,6
->F-1,9
Example sequence of a read call, user presses Ctrl-C before syscall on host is called:
<-Fread,3,1234,6
->F-1,4,C
<-T02
Example sequence of a read call, user presses Ctrl-C after syscall on host is called:
<-Fread,3,1234,6
->X1234,6:XXXXXX
<-T02
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